1. This invention relates in general to offshore drilling and production equipment, and in particular for treating produced water from a subsea well.
Well fluid produced from a subsea well typically includes liquid hydrocarbons or oil, gaseous hydrocarbons or natural gas, and water. Transporting water from a subsea well decreases the transportation efficiency and increases the reservoir energy requirements and size of the pump (if used) required to pump the well fluid from the subsea well to a processing facility or to a collection manifold. Typically the processing facility is either on a platform or on land. Further, water in the hydrocarbon stream increases the risk of hydrates and the demand for chemicals to control hydrates.xe2x80x9d
There is a pilot program in which a subsea separator is placed adjacent a subsea well that separates the produced water from the well fluid. The produced water, which typically includes some residual gaseous and liquid hydrocarbon, is then reinjected into another subsea well. The hydrocarbons exiting the subsea separator are pumped to a fully manned processing facility on a platform. After processing on the platform, the hydrocarbon is conveyed to a transport means. In the pilot program, there must be a pump capable of pumping the oil and gas from the subsea separator to a fully-manned processing facility. Additionally, the water with residual hydrocarbons must be reinjected into a subsea well because it is too contaminated to be released or dumped to sea. Furthermore, reinjecting water into a subsea well can be expensive and is not always feasible; subject to the availability of a suitable subsea reservoir.
A method and system for separating and treating water produced from a subsea well includes separation of the water from the well fluid at a subsea separator and further separation of the water from residual hydrocarbons on a vessel at the sea surface. The vessel is preferably an unmanned, or not normally manned buoy. The well fluid that contains oil, natural gas, and water is conveyed to the subsea separator where the water is removed and the oil and gas, or produced hydrocarbons, are conveyed to a subsea gathering facility for collection and processing at a facility away from the subsea well. The water removed from the subsea separator, or xe2x80x9cdirty water,xe2x80x9d typically has residual gaseous and sometimes liquid hydrocarbons. The dirty water is pumped to the floating vessel at the surface where the water enters a surface separator. There can be a plurality of individual separators for removing the residual hydrocarbons from the dirty water.
The water exiting from the surface separator, or treated water, is sufficiently clean to be dumped to the sea. Alternatively, the treated water can be combined with sea water that is being injected into another subsea well during well flooding operations. Any liquid residual hydrocarbons, or oil, from the surface separator can be pumped back subsea for collection and processing with the other produced hydrocarbons. The gaseous residual hydrocarbons, or natural gas, can also be transported subsea for further collection and processing with the other produced hydrocarbons. The gaseous residual hydrocarbons can be compressed in order to convey the gaseous residual hydrocarbons subsea, or the gaseous residual hydrocarbons can be mixed with sea water to form a hydrate slurry that is capable of being pumped subsea. Alternatively, the gaseous residual hydrocarbons at the surface vessel can be used as a fuel for gas powered equipment on the vessel or buoy. The gas powered equipment can be used to drive various rotating machinery and generators for providing electricity to the vessel or buoy. Gaseous hydrocarbons from the subsea separator can be pumped with the dirty water or separately to the vessel if more gaseous hydrocarbons are needed to fuel the gas powered equipment.